ICs as semiconductor devices which also include transistors and LSIs in their category are typically fabricated by the following steps:
(a) provide a lead frame material that is made of a strip of Cu alloy having a thickness of 0.1-0.3 mm; PA1 (b) etch or stamp out a lead frame conforming to the shape of the ICs to be fabricated; PA1 (c) apply high-purity Si or Ge semiconductor chips to selected areas in the lead frame by thermocompression with an electrically conductive resin such as Ag paste, or by soldering with a plating of Au, Ag, Ni, Cu or an alloy thereof formed on the mating surfaces of each semiconductor chip and the lead frame, or by Au brazing; PA1 (d) connect each semiconductor chip to the lead frame by bridging them with very fine Au bonding wires; PA1 (e) enclose with a protective plastic package the semiconductor chips, gold wires and parts of the lead frame to which the chips have been bonded; and PA1 (f) cut the lead frame into discrete ICs.
The very fine Au wires conventionally employed as bonding wires in the fabrication of semiconductor devices are expensive and increasing attention is being drawn to using less expensive oxygen-free copper in the form of a very fine wire. However, an ingot of the conventional oxygen-free copper is fairly hard (Hv: 50-60), so if a very fine wire drawn from this ingot is applied to the Si semiconductor chip in the bonding step, the ball forming at the wire tip may frequently damage the wiring Al alloy coat on the chip, or may even sometimes cause microcracking in the chip. A further problem results from the low elongation of the very fine oxygen-free Cu wire: not only is considerable difficulty encountered with forming and retaining the proper shape of a wire loop at bonding but also the wire has a great tendency to break without being connected to the appropriate area in the lead frame.